This invention relates to a semiconductor element of a high voltage applied to, for example, a MOS transistor, an IGBT (Insulated Gate Bipolar Transistor), etc., and more particularly to a semiconductor device of a junction terminal structure and its manufacturing method.
In semiconductor devices of a high voltage, a high electric field may be locally generated and breakdown may occur depending upon the shape of a junction or by the influence of an external charge. To prevent such breakdown, a semiconductive film such as a polysilicon layer is formed on the surface of a semiconductor area of a low impurity density which will serve as a depletion layer.
As shown in FIG. 7, first, ion implantation and diffusion is executed in, for example, an n-type semiconductor substrate 11. As a result, for example, a p-type anode layer 12 is selectively formed in a surface of the semiconductor substrate 11, and, for example, an n.sup.+ -type channel stopper layer 13 is selectively formed in the same surface of the substrate 11 with a predetermined distance from the anode layer 12. Further, an n.sup.+ -type cathode layer 14 is formed on the reverse surface of the semiconductor substrate 11.
Subsequently, a thermally oxidized film (not shown) is formed on the semiconductor substrate 11 by thermal oxidation. Then, that portion of the thermally oxidized film, which is located between the anode layer 12 and the channel stopper layer 13, is etched.
After that, a semiconductive film 15 with a thickness of, for example, 1.5 .mu.m is formed by low pressure CVD on the entire top surface of the resultant structure. Then, those portions of the semiconductive film 15, which are located on the anode layer 12 and the channel stopper layer 13, are etched, and the semiconductive film on the reverse surface is selectively etched.
Thereafter, an oxide film 17 is formed on the entire top surface of the resultant structure by atmospheric pressure CVD. Then, those portions of the thermally oxidized film and the oxide film 17, which are located on the anode layer 12 and the channel stopper layer 13, are etched.
In the next stage, a metallic film formed of, for example, aluminum is provided on the entire top surface. This metallic film is then selectively etched so as to expose the surface of the oxide film 17. As a result, an anode electrode 18 connected to the anode layer 12 and a channel stopper electrode 19 connected to the channel stopper layer 13 are formed.
Lastly, a cathode electrode 20 made of, for example, aluminum is formed on the reverse surface of the semiconductor substrate 11.
The semiconductive film 15 is formed by low pressure CVD and mixed with oxygen, for example, of a predetermined density. However, since the film is formed by a high temperature treatment, it is in a state in which oxygen is liable to diffuse until, for example, it reaches a density higher than a predetermined value and a position out of a predetermined range. Accordingly, while transferring it from the furnace to the outside air, oxygen is absorbed from the atmosphere and diffuses to a deep portion of the semiconductive film 15 heated to a high temperature, whereby the semiconductive film 15 inevitably has an area of high oxygen concentration. As a result, as is shown in FIG. 8, the area in which the oxygen concentration varies extends from the surface of the semiconductive film 15 to a depth of about 1 .mu.m, whereas an area in which the oxygen concentration is constant extends over a length of only 0.5 .mu.m in the depth direction.
Therefore, when implanting carriers into the semiconductive film 15, they are trapped in a high oxygen concentration area located at an upper portion of the film 15. The charge accumulated in this area disturbs an electric field generated in the device under the semiconductive film, thereby degrading the breakdown voltage. Furthermore, since the semiconductive film 15 has a small area of constant oxygen concentration, the device shows a high resistivity.